Flash memory device and method for fabricating the same, and programming and erasing method thereof

ABSTRACT

A flash memory device of SONOS structure and a method for fabricating the same, and programming and erasing operation methods, to improve reliability such as endurance and retention, are disclosed, which includes a first conductive type semiconductor substrate; an ONO layer on the semiconductor substrate; a first control gate on the ONO layer; second and third control gates on the ONO layer at both sides of the first control gate; and source and drain regions in the surface of the semiconductor substrate at both sides of the second and third control gates.

This application claims the benefit of the Korean Application No.P2004-28282 filed on Apr. 23, 2004, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flash memory device, and moreparticularly, to a flash memory device having an SONOS(Polysilicon-Oxide-Nitride-Oxide-Semiconductor) structure and a methodfor fabricating the same, and programming and erasing methods thereof,to improve reliability such as endurance and retention.

2. Discussion of the Related Art

A typical example of a nonvolatile memory device, in which data is noterased even though a voltage is not applied, is EEPROM (ElectricallyErasable Programmable Read Only Memory). Generally, the EEPROM takes ascheme using a floating gate-type cell. With rapid development ofhigh-integration device, it necessarily requires the decrease in a sizeof the floating gate-type cell according to the related art. However, itis impossible to decrease the cell size since it requires a high voltageon programming and erasing modes and it is difficult to obtain themargin of process for defining tunneling. In this reason, variousnonvolatile memory devices such as SONOS, FeRAM, SET and NROM have beenstudied actively as the substitute for the floating gate-type cell.Among them, SONOS cell has attracted great attentions as the nextnonvolatile memory device, which can substitute for the stacked floatinggate-type cell.

Hereinafter, a related art SONOS-type flash memory device will bedescribed with reference to the accompanying drawings. FIG. 1 is across-sectional view illustrating a unit cell of a related artSONOS-type flash memory device.

As shown in FIG. 1, the SONOS-type flash memory device according to therelated art includes a p-type semiconductor substrate 11, an ONO layer18, a control gate 15, and source and drain regions 16 and 17. At thistime, the ONO layer 18 is formed in a method of sequentially stacking afirst oxide layer 12, a nitride layer 13 and a second oxide layer 14.Then, the control gate 15 is formed on the ONO layer 18, and the sourceand drain regions 16 and 17 are formed in a method of implanting highlydoped n-type impurity ions into the surface of the semiconductorsubstrate 11 at both sides of the control gate 15. At this time, thefirst oxide layer 12 acts as a tunneling oxide layer, and the nitridelayer 13 acts as a memory layer by controlling a threshold voltage Vthby charging electric charge to a trap site or discharging the electriccharge. Also, the second oxide layer 14 acts as a blocking oxide layerpreventing loss of the electric charge.

In the aforementioned SONOS-type flash memory device according to therelated art, a programming operation uses a CHEI (Channel Hot ElectronInjection) method, and an erasing operation uses an HHI (Hot HoleInjection) method to remove injected electrons.

On the programming operation, a predetermined positive (+) voltage isapplied to the drain region 17 and the control gate 15, and the sourceregion 16 and the semiconductor substrate (body) are ground. Under thiscondition, according as a bias is applied, channel electrons areaccelerated by a lateral electric field formed from the source region 16to the drain region 17, whereby the channel electrons become hotelectrons around the drain region 17. Also, the hot electrons jump overa potential barrier of the first oxide layer 12, and are locally trappedto a trap level of the nitride layer 13 around the drain region 17,thereby increasing the threshold voltage. This programming method isreferred to as CHEI (Channel Hot Electron Injection).

On the erasing operation, a predetermined positive (+) voltage isapplied to the drain region 17, and a predetermined negative (−) voltageis applied to the control gate 15. Also, the source region 16 and thesemiconductor substrate (body) 11 are ground. Under this condition,according as a bias is applied, a depletion region is formed in then-type drain region 17 by a high electric field formed in an overlaparea between the drain region. 17 and the control gate 15. In thedepletion region, pairs of electron and hole are generated by band toband tunneling. Then, the electron escapes to the n-type region, and thehole is accelerated by a lateral electric field of the depletion region,whereby the hole is changed to a hot hole. The hot hole jumps over anenergy barrier between the first oxide layer 12 and the semiconductorsubstrate 11, injected and trapped to a valance band of the nitridelayer 13, thereby performing the erasing operation lowing the thresholdvoltage. This erasing method is referred to as HHI (Hot Hole Injection).

Also, the aforementioned SONOS-type flash memory device records 2-bitdata in one cell. That is, the programming operation of the related artSONOS-type flash memory device uses the CHEI (Channel Hot ElectronInjection) method, and the erasing operation thereof uses the HHI (HotHole Injection) method. By applying the voltage of the aforementionedcondition, the electron is trapped to the nitride layer 13 around thedrain region 17, thereby storing 1-bit data. Then, a predeterminedpositive (+) voltage is applied to the source region 16 and the controlgate 15, and the drain region 17 and the semiconductor substrate (body)are ground, whereby hot electrons are generated around the source region16. The hot electrons jump over the potential barrier of the first oxidelayer 12, and are trapped to the nitride layer 13 around the sourceregion 16, thereby storing 1-bit data.

In the programming and erasing operations, if the electron and hole areshallowly trapped to the nitride layer 13, program and erase cyclingbecomes low. Accordingly, the technology for overcoming this problem hasbeen published in IEDM document (Novel Operation Schemes to ImproveDevice Reliability in a Localized Trapping Storage SONOS-Type FlashMemory, 2003).

On the programming operation, when the hot electrons jump over thepotential barrier of the first oxide layer 12, and are trapped to thenitride layer 13 around the drain region 17, some of the hot electronsare deeply trapped to the nitride layer 13, and the other hot electronsare shallowly trapped to the nitride layer 13. Among them, the hotelectrons shallowly trapped to the nitride layer 13 escape during astorage period due to a damage of the first oxide layer generated by theprogram and erase cycling, thereby generating loss of the electriccharge.

According to IEDM document published in 2003, it is proposed to carryout an electrical anneal after Hot Electron Injection of the programmingoperation or after Hot Hole Injection of the erasing operation, toimprove over-erase in state of the low threshold voltage, and to preventloss of the electric charge in state of the high threshold voltage. Onthe erasing operation, after hole injection, the electron and hole arerecombined in the erasing electrical anneal. On the programmingoperation, after electron injection, the electron shallowly trapped tothe nitride layer is removed, and the electron deeply trapped to thenitride layer remains by the programming electrical anneal. In theerasing electrical anneal, as shown in FIG. 1, the source region 16 andthe drain region 17 are ground, and the pulse (10 ms) of 10V is appliedto the control gate 15. In the programming electrical anneal, the sourceregion 16 and the drain region 17 are ground (0V), and the pulse (10 ms)of −4V is applied to the control gate 15.

However, the aforementioned IEDM document has the following problems.

On the programming operation, the electrons shallowly trapped to thenitride layer are removed by the programming electrical anneal, therebydecreasing loss of the electric charge. After programming, it isimpossible to remove the hopping electrons moved (spread) to a lateralside by the programming electrical anneal. Accordingly, even in case ofHot Hole Injection in a large amount on the erasing operation, thetrapped electrons are not removed completely, thereby degradingreliability. Especially, when separately programming data to the nitridelayer around the source region and the drain region, for example,programming 2-bit data in one cell, the aforementioned problem becomesmore serious.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a flash memory deviceand a method for fabricating the same, and programming and erasingmethods that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide a flash memory deviceand a method for fabricating the same, and programming and erasingmethods, to completely remove hopping electrons moved (spread) to alateral direction, after programming.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, aflash memory device includes a first conductive type semiconductorsubstrate; an ONO layer on the semiconductor substrate; a first controlgate on the ONO layer; second and third control gates on the ONO layerat both sides of the first control gate; and source and drain regions inthe surface of the semiconductor substrate at both sides of the secondand third control gates.

Furthermore, the flash memory device includes an insulating layer toinsulate the first control gate from the second and third control gates.

Furthermore, the flash memory device includes a planarization insulatinglayer on an entire surface of the semiconductor substrate, and havingrespective contact holes in the first, second and third control gatesand the source and drain regions; and a word line, first and secondcontrol lines, and first and second bit lines respectively connectedwith the first, second and third control gates and the source and drainregions through the respective contact holes.

In another aspect, a method for fabricating a flash memory deviceincludes the steps of forming an ONO layer having a stacked structure ofa first oxide layer, a nitride layer and a second oxide layer on asemiconductor substrate on which an active region is defined; forming afirst control gate on the ONO layer; forming an insulating layer on thesurface of the first control gate; forming second and third controlgates on the ONO layer at both sides of the first control gate; andforming source and drain regions on the semiconductor substrate at bothsides of the second and third control gates.

At this time, the second oxide layer is formed in a method of oxidizingthe nitride layer.

Also, the insulating layer is formed in a method of oxidizing theexposed surface of the first control gate.

Also, the source and drain regions are formed in a method of implantinghighly doped n-type impurity ions by using the first, second and thirdcontrol gates as a mask.

Furthermore, the method includes the steps of forming a planarizationinsulating layer on the entire surface of the semiconductor substrateincluding the first, second and third control gates; forming respectivecontact holes by selectively removing the planarization insulating layerand the ONO layer to expose the first, second and third control gatesand the source and drain regions; and forming a word line, first andsecond control lines, and first and second bit lines respectivelyconnected with the first, second and third control gates and the sourceand drain regions through the respective contact holes.

At this time, the planarization insulating layer is formed of BPSG.

In another aspect, a programming method of a flash memory device ofSONOS structure, the flash memory device including second and thirdcontrol gates at both sides of a first control gate, source and drainregions in a surface of a semiconductor substrate at both sides of thesecond and third control gates, and ONO layers between the control gatesand the semiconductor substrate, includes a first step of applying ahigh voltage pulse to the first and third control gates, and trappinghot electrons to the ONO layer around the drain region; and a secondstep of annealing the trapped electrons by applying a high voltage pulseto the first control gate.

In the first step, 0V is applied to the source region, a voltage over athreshold voltage is applied to the second control gate, a voltage pulseof 10V or more is applied to the first and third control gates, and avoltage of 4V or more is applied to the drain region.

In the second step, 0V is applied to the source and drain region, avoltage of −4V is applied to the second and third control gates, and avoltage pulse of −10V is applied to the first control gate.

Furthermore, the method includes a third step of trapping hot electronsto the ONO layer around the source region by applying a high voltagepulse to the first and second control gates.

In another aspect, an erasing method of a flash memory device of SONOSstructure, the flash memory device including second and third controlgates at both sides of a first control gate, source and drain regions ina surface of a semiconductor substrate at both sides of the second andthird control gates, and ONO layers between the control gates and thesemiconductor substrate, includes a first step of injecting a hole tothe ONO layer around the source or drain region by applying a negativevoltage to the second or third control gate; and a second step ofannealing the injected hole applying a positive high voltage pulse tothe first control gate, and applying a negative high voltage pulse tothe second and third control gates.

In the first step, a voltage over 0V or 5V is applied to the source ordrain region, a voltage over a threshold voltage is applied to thesecond or third control gate, and a voltage pulse of −3V or more isapplied to the first/third or second control gate.

In the second step, 0V is applied to the source and drain regions, avoltage pulse of 10V is applied to the second and third control gates,and a voltage pulse (several tens Ms) of −10V is applied to the firstcontrol gate, thereby removing electrons moved to a lateral side.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a cross-sectional view illustrating a related art SONOS-typeflash memory device;

FIG. 2 is a cross-sectional view illustrating a flash memory deviceaccording to the present invention; and

FIG. 3A to FIG. 3H are cross-sectional views illustrating thefabrication process of a flash memory device according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Hereinafter, a flash memory device and a method for fabricating thesame, and programming and erasing methods will be described withreference to the accompanying drawings.

FIG. 2 is a cross-sectional view illustrating a flash memory deviceaccording to the present invention. In the flash memory device accordingto the present invention, as shown in FIG. 2, an ONO layer 28 is formedon a p-type semiconductor substrate 21, wherein the ONO layer 28 isformed by sequentially stacking a first oxide layer 22, a nitride layer23, and a second oxide layer 24. Then, a first control gate 25 is formedon the ONO layer 28, and second and third control gates 29 and 30 areformed on the ONO layer 28 at both sides of the first control gate 25.After that, source and drain regions 26 and 27 are formed in the surfaceof the p-type semiconductor substrate 21 at both sides of the second andthird control gates 29 and 30 by implantation of n-type impurity ions.

At this time, the first control gate 25 is isolated from the second andthird control gates 29 and 30 by a third oxide layer 31. Then, aplanarization insulating layer 32 is formed on an entire surface of thesemiconductor substrate 21 including the first, second and third controlgates 25, 29 and 30. Also, contact holes are respectively formed in thefirst, second and third control gates 25, 29 and 30 and the source anddrain regions 26 and 27. In this state, a word line W/L and first andsecond control lines CL1 and CL2 are connected with the first, secondand third control gates 25, 29 and 30 through the respective contactholes. Also, the source and drain regions 26 and 27 are connected with afirst bit line BL1 and a second bit line BL2.

Also, the first oxide layer 22 acts as a tunneling oxide layer. Thenitride layer 23 acts as a memory layer to charge the electric charge ina trap site, or to discharge the electric charge, whereby the nitridelayer 23 has a memory function by controlling a threshold voltage Vth.Meanwhile, the second oxide layer 24 acts as a blocking oxide layer toprevent loss of the electric charge.

A method for fabricating the aforementioned flash memory deviceaccording to the present invention will be described as follows. FIG. 3Ato FIG. 3H are cross-sectional views illustrating the fabricationprocess of the flash memory device according to the present invention.

As shown in FIG. 3A, a p-type semiconductor substrate 21 is defined as afield region and an active region. Then, a field oxide layer (not shown)is formed in the field region of the p-type semiconductor substrate 21,and a buffer oxide layer 33 is formed on the p-type semiconductorsubstrate 21. Also, the process for ion implantation is progressed toform a well region (not shown) in the active region of the semiconductorsubstrate 21 and to control a threshold voltage.

Referring to FIG. 3B, after removing the buffer oxide layer 33, a firstoxide layer 22, a nitride layer 23 and a second oxide layer 24 aresequentially deposited on an entire surface of the semiconductorsubstrate 21 to form an ONO layer 28, wherein the first oxide layer 22acts as a tunneling insulating layer, the nitride layer 23 locally trapselectrons, and the second oxide layer 24 acts as a blocking layer. Then,a first polysilicon layer 25a is formed on the ONO layer 28. At thistime, the second oxide layer 24 may be formed in a method of depositingan oxide layer by CVD, or oxidizing the nitride layer 23.

As shown in FIG. 3C, a photosensitive layer (not shown) is formed on thefirst polysilicon layer 25 a, and then the exposure and developmentprocess using a mask is carried out thereto, whereby the firstpolysilicon layer 25 a is selectively removed to form a first controlgate 25. Thereafter, the photosensitive layer is removed. Afterpatterning the first control gate 25, the cleaning process is carriedout.

After that, as shown in FIG. 3D, the exposed surface of the firstcontrol gate 25 is oxidized, thereby forming an oxide layer 31 coveringthe side portions and the upper surface of the first control gate 25.Then, as shown in FIG. 3E, a second polysilicon layer (not shown) isformed on the entire surface of the semiconductor substrate 21 includingthe oxide layer 31 and the first control gate 25, and then theanisotropic etching process (etch-back process) is carried out, therebyremoving the second polysilicon layer, and forming second and thirdcontrol gates 29 and 30 on the ONO layer 28 in the side portions of theoxide layer 31 at both sides of the first control gate 25.

Referring to FIG. 3F, highly doped n-type impurity ions are implantedinto the semiconductor substrate 21 by using the first, second and thirdcontrol gates 25, 29 and 30 as a mask, thereby forming source and drainregions 26 and 27 in the surface of the semiconductor substrate 21 atthe side portions of the second and third control gates.

As shown in FIG. 3G, a planarization insulating layer 32 of BPSG isdeposited on the entire surface of the semiconductor substrate 21including the first, second and third control gates 25, 29 and 30. Then,as shown in FIG. 3H, the planarization insulating layer 32 and the ONOlayer 28 are selectively removed to expose the first, second and thirdcontrol gates 25, 29 and 30 and the source and drain regions 26 and 27by photolithography, thereby forming contact holes. After that, a metallayer is deposited on the entire surface of the semiconductor substrate,and selectively removed, whereby a word line WL and first, secondcontrol lines CL1 and CL2 and first and second bit lines BL1 and BL2 arerespectively connected with the first, second and third control gates25, 29 and 30, and the source region 26 and the drain regions 27 throughthe respective contact holes.

Hereinafter, programming/erasing and reading operations of theaforementioned flash memory device will be described in detail.

On the data programming operation in the flash memory device,programming and programming electrical anneal are repetitivelyperformed. That is, 0V is applied to the first bit line BL1, a voltageover the threshold voltage is applied to the first control line CL1, avoltage pulse (several tens μs) over 10V is applied to the word line W/Land the second control line CL2, and a voltage over 4V is applied to thesecond bit line BL2, whereby channel electrons are accelerated by alateral electric field formed from the source region 26 to the drainregion 27, thereby generating hot electrons around the drain region 27.The hot electrons jump over the potential barrier of the first oxidelayer 22, and then the hot electrons are locally trapped to a trap levelof the nitride layer 23 around the drain region 27.

Also, the programming electrical anneal is performed to trap electronsto the nitride layer 23, and to remove the electrons moved to thelateral side among the trapped electrons. That is, 0V is applied to thefirst and second bit lines BL1 and BL2, the voltage pulse (several tensμs) of −4V is applied to the second control line CL2, and the voltagepulse (several tens ms) of −10V is applied to the word line W/L, so thatthe electrons moved to the lateral side are removed. At this time, evenif the voltage pulse (several tens ms) of −10V is applied to the wordline W/L, it is possible to remove only the electrons moved to thelateral side without the effect on the electrons trapped to the nitridelayer 23 around the drain region 27, as shown in FIG. 2, since thevoltage pulse of −4V is applied to the third control gate 30.

As described above, to program 2-bit data in a unit cell, it is requiredto trap the electrons to the insulating layer 23 around the drain region27, to apply the voltage over 4V to the first bit line BL1, to apply thevoltage over the threshold voltage to the second control line CL2, toapply the voltage pulse (several tens μs) over 10V to the word line W/Land the first control line CL1, and to apply 0V to the second bit lineBL2. As a result, the channel electrons are accelerated by the lateralelectric field formed from the drain region 27 to the source region 26,whereby the channel electrons around the source region 26 become the hotelectrons. The hot electrons jump over the potential barrier of thefirst oxide layer 22, so that the hot electrons are locally trapped tothe trap level of the nitride layer 23 around the source region 26,thereby programming 1-bit data.

If 2-bit data is programmed, in which the electrons are separatelytrapped to the nitride layer 23 around the source and drain regions 26and 27, the programming electrical anneal is performed as follows. Thatis, 0V is applied to the first and second bit lines BL1 and BL2, thevoltage pulse (several tens μs) of −4V is applied to the first andsecond control lines CL1 and CL2, and the voltage pulse (several tensms) of −10V is applied to the word line W/L, thereby removing theelectrons moved to the lateral side. At this time, even if the voltagepulse (several tens ms) of −10V is applied to the word line W/L, it ispossible to remove the electrons (the electrons trapped to the lowerside of the first control gate) moved to the lateral side without theeffect on the electrons trapped to the nitride layer 23 around thesource region 26 and the drain region 27, since the voltage pulse of −4Vis applied to the second and third control gates 29 and 30.

On the data erasing operation of the flash memory device, erasing anderasing electrical anneal are repetitively performed. That is, 0V isapplied to the first bit line BL1, the voltage over the thresholdvoltage Vth is applied to the first control line CL1, the voltage pulse(several tens is) over −3V is applied to the word line W/L and thesecond control line CL2, and the voltage of SV or more is applied to thesecond bit line BL2. As a result, a depletion region is formed in thedrain region 27 highly-doped with n-type impurity ions by a highelectric field formed in an overlap region between the drain region 27and the third control gate 30. In the depletion region, pairs ofelectron and hole are formed by band to band tunneling. The electronescapes to the highly doped n-type impurity region. Also, the hole isaccelerated by the lateral electric field formed in the depletionregion, and changed to the hot hole, whereby the hot hole jumps over anenergy barrier between the first oxide layer 12 and the semiconductorsubstrate 11, and is injected to a valance band, whereby the erasingoperation is completed.

To remove the electrons moved to the lateral side, the erasingelectrical anneal is performed. That is, the voltage of 0V is applied tothe first and second bit lines BL1 and BL2, the voltage pulse (severaltens is) of 10V is applied to the first and second control lines CL1 andCL2, and the voltage pulse (several tens ms) of −10V is applied to theword line W/L, thereby removing the electrons moved to the lateral side.

As mentioned above, the flash memory device and the programming anderasing methods according to the present invention have the followingadvantages.

The second and third control gates are formed at both sides of the firstcontrol gate. Then, in state of applying the predetermined voltage tothe second and third control gates for maintaining the programmedelectrons, and applying the high voltage to the first control gate, theelectrical annealing is progressed to remove the trapped electrons movedto the lateral side, thereby improving reliability of the flash memorydevice.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A flash memory device comprising: a first conductive typesemiconductor substrate; an ONO layer on the semiconductor substrate; afirst control gate on the ONO layer; second and third control gates onthe ONO layer at both sides of the first control gate; and source anddrain regions in the surface of the semiconductor substrate at bothsides of the second and third control gates.
 2. The flash memory deviceof claim 1, further comprising an insulating layer to insulate the firstcontrol gate from the second and third control gates.
 3. The flashmemory device of claim 1, further comprising: a planarization insulatinglayer on an entire surface of the semiconductor substrate, and havingrespective contact holes in the first, second and third control gatesand the source and drain regions; and a word line, first and secondcontrol lines, and first and second bit lines respectively connectedwith the first, second and third control gates and the source and drainregions through the respective contact holes. 4-16. (canceled)